This invention relates to the field of hollow fibers, especially to asymmetric hollow fibers comprising SIXEF.TM.-Durene polyimide (the polymerization product of 2,2-bis[3,4-dicarboxyphenyl] hexafluoropropane dianhydride and 2,3,5,6-tetramethylphenylene diamine monomers).
In creating a gas or liquid separation membrane, it is desirable to have both a high rate of permeation or throughput and a high separation factor. This combination of characteristics permits the effective separation of a relatively large volume of fluid per unit time. Large rates of permeation also allow the membrane to be operated at lower pressures, which improves cost-effectiveness and reduces wear and tear.
Highly porous membranes tend to be very permeable, but do a poor job of separating fluids into their components. Less porous, dense membranes can be more selective, but at the cost of reduced throughput. Asymmetric membranes represent a compromise between the two in that they generally have a thin, dense separation layer and a less dense, more permeable layer.
Asymmetric membranes are often chosen for separation applications because they may provide a good combination of permeation and separation. Hollow fibers having a dense skin and a porous body can be used to make such membranes. However, such fibers generally need to be coated with another highly permeable substance to achieve acceptable selectivities.
U.S. Pat. No. 4,705,540 issued to Hayes describes polyimide gas separation membranes made by casting a polyimide solution on a plate. The polyimides described by Hayes include aromatic fluoropolymers.
U.S. Pat. No. 4,871,494 issued to Kesting, et al. describes a process for forming asymmetric gas separation membranes having graded density skins. This process comprises dissolving a hydrophobic polymer in a Lewis acid:base solvent system wherein the Hildebrand parameters of the solvent species and the polymer are within less than 1.5, creating a dope from this solution, forming the dope into an appropriate shape, coagulating the dope, desolvating, washing and drying. According to the patent, the resultant membrane has increased free volume as evidenced by a glass transition temperature greater than the bulk glass transition temperature of the polymer.
U.S. Pat. No. 4,881,954 issued to Bikson, et al. describes permeable membranes for enhanced gas separation in which an asymmetric porous support is coated with a separation layer. The support is made asymmetric prior to coating to provide mechanical stability and a uniform support surface.
U.S. patent application No. 686,739 filed on Apr. 17, 1991 by Chung, et al. describes a method for making asymmetric hollow fibers from fluoropolymers. The outer layer of these fibers is coated with a highly permeable substance to achieve high selectivities.